Mypy: Class __init__ "Too few arguments" should be a better error message

[indigoviolet]

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?

Feature request

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

class Foo:
    def __init__(self, bar: int):
        pass


f = Foo()

• What is the actual behavior/output?

➜ python test.py
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    f = Foo()
TypeError: __init__() missing 1 required positional argument: 'bar'

~ via ⬢ 13.8.0 via 🐍 3.8.1 
➜ mypy --pretty test.py
test.py:6: error: Too few arguments for "Foo"
    f = Foo()
        ^
Found 1 error in 1 file (checked 1 source file)

• What is the behavior/output you expect?

mypy should say which arguments are missing, like Python does. This was fixed for functions in #505, but it doesn't seem to work for class __init__.

• What are the versions of mypy and Python you are using?

0.782, 3.8.1

• Do you see the same issue after installing mypy from Git master?

I have not tested this.

• What are the mypy flags you are using? (For example --strict-optional)

--pretty

• If mypy crashed with a traceback, please paste
  the full traceback below.

(You can freely edit this text, please remove all the lines
you believe are unnecessary.)

[abhisht51]

Hi,
This would be my first time contributing to this repo. Can I take this up ?

[gvanrossum]

Sure, give it a try. Read CONTRIBUTING.md to get started.

[Shkurupii]

I took a look at this issue recently. The problem is reproduced not only on "init" but for empty function as well.
With next samples:

a = 2
a.__pow__()


class Foo:
    def __init__(self, bar: int):
        pass


c = Foo()


def foo(baz: int, bas: int):
    pass


foo()

mypy returns:

(mypy) ➜  mypy git:(master) ✗ mypy samples-9340.py
samples-9340.py:2: error: Too few arguments for "__pow__" of "int"
samples-9340.py:10: error: Too few arguments for "Foo"
samples-9340.py:17: error: Too few arguments for "foo"
Found 3 errors in 1 file (checked 1 source file)

As I understand, the function doesn't enter in that main "if" because argument_names is [], subsequently, the length is less then 1, "all" returns False as well. My proposal https://github.com/Shkurupii/mypy/commit/0c759bcdb5c569242aeb4a56e20d7743fcf5a520 is to leave a check only for argument_names is None only. The mypy in this way returns errors similar to the python interpreter.
Basically, the idea is to treat everything in "Missing positional argument" way no matter zero arguments passed or more.

(mypy) ➜  mypy git:(issue-9340) ✗ mypy samples-9340.py 
samples-9340.py:2: error: Missing positional argument
samples-9340.py:10: error: Missing positional argument "bar" in call to "Foo"
samples-9340.py:17: error: Missing positional arguments "baz", "bas" in call to "foo"
Found 3 errors in 1 file (checked 1 source file)

But in that case, it requires a lot of refactoring over tests, 197 tests are broken.
@gvanrossum what do you think? give me a few words of advice

[gvanrossum]

I'm sorry, I don't have time right now.

[Shkurupii]

No problem.
@JukkaL maybe you suggest smth.

[Shkurupii]

@ilevkivskyi, maybe you can suggest

[gvanrossum]

Maybe try a simpler issue rather than spamming all the devs.